JPH1160789A - Production of microporous film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain the subject film capable of regulating permeability without spoiling strengths, while excellent in dimensional stability at high temperatures and used for battery separators, etc., by using a pre-extraction drawing together with a post-extraction drawing and performing a heat treatment. SOLUTION: This production of microporous film is to melt and knead a composition comprising a polyolefin resin and a plasticizer, form the composition in a sheet form by extruding and cooling to solid, draw at least one time in at least one axial direction, extract the plasticizer, successively draw at least one time in at least one axial direction and perform a heat treatment. In the heat treatment, it is preferable to bring the temperature to equal to or higher than a temperature 50 deg.C lower than the melting point Tm of the obtained film to less than the Tm and heat relax the film by bringing the relaxing rate to 1-50%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention provides a means suitable for producing separators used for battery materials such as various cylindrical batteries, prismatic batteries, thin batteries, button batteries, electrolytic capacitors and the like. Things.

[0002]

2. Description of the Related Art Microporous membranes have been used as materials for filter media for water purifiers and the like, for breathable clothing, separators for batteries and separators for electrolytic capacitors. In recent years, demand for lithium-ion secondary battery applications has been particularly growing, and as the energy density of batteries has increased, high performance has also been required for separators.

[0003] Since a lithium ion secondary battery uses chemicals such as an electrolytic solution and positive and negative electrode active materials, a polyolefin polymer is generally used for the material of the separator in consideration of chemical resistance. Particularly, inexpensive polyethylene and polypropylene are used. For separators for non-aqueous electrolyte batteries such as lithium ion secondary batteries, electrode short circuit prevention function, high ion permeability, assembly workability when winding the battery, battery safety, reliability, etc. It has been demanded as more basic performance. Furthermore, in recent years, in order to meet the needs of diversified battery grades, there is an urgent need to develop a technology for freely adjusting the permeation performance such as a pore structure and a porosity and a technology for adjusting the dimensional stability at high temperatures.

The function of preventing an electrode short-circuit means that a separator functions as a partition wall interposed between the positive and negative electrodes to prevent an internal short-circuit. In order to prevent an internal short circuit, the separator needs to have high strength, a small pore diameter, and an appropriate film thickness. In a secondary battery, an internal electrode expands due to charge and discharge, and in some cases, a pressure of several tens of kg / cm ² may be applied to the separator. In addition, the electrode surface is not always smooth, and active material particles of various sizes are formed as projections. Even in such a case, the separator is required to have high strength that does not break. When the separator is used for a rectangular battery or a thin battery, the coil formed by laminating the electrode and the separator is compressed and casing, so that the demand for high strength can be said to be even stronger.

[0005] The term "ion-permeable" means the ability of a separator to transmit only ions or an electrolytic solution without transmitting active material particles. Generally, in order to reduce ohmic loss and increase discharge efficiency, performance such as high porosity, low air permeability, and low electric resistance is required. However, in recent years, for applications that do not require a large current discharge or for applications that have particularly high demands on battery safety, there are cases where a dense separator that plays a role against the high permeability is required. Therefore, it is useful to develop a flexible molding technique that can freely adjust the permeability for any request.

[0006] Regarding the assembling processability, when a separator is wound with an electrode while applying a constant tension in the machine direction,
It is required that the separator does not extend in the machine direction and that the dimension does not change in the width direction, and a high elastic modulus is required.
Battery safety is a function to prevent battery runaway and explosion by automatically shutting off the current and stopping heat generation when the battery heats up due to external short circuit or overcharge, etc. Means When the temperature inside the battery rises to near the melting point of the resin that makes up the separator,
The separator exerts a so-called shutdown function by closing pores by heat flow, thermal deformation or heat shrinkage, or by absorbing resin on the electrode surface to form an insulating film. The lower the temperature at which this function is exhibited, the more desirable it is because it has the ability to cut off current at a low temperature and suppress heat generation. Further, the wider the temperature region in the shutdown state is, the longer the time during which the current is cut off is. Therefore, it is desirable to be able to withstand a temperature rise due to more intense heat generation.

The dimensional stability at high temperature is to evaluate the degree of dimensional deformation of the separator due to thermal shrinkage or the like, assuming that some temperature treatment is performed during battery manufacture or that the inside of the battery is heated due to trouble. . In the former case, if the separator shrinks and deforms in the width direction, the electrodes will be exposed and short-circuited internally, and if shrinkage stress acts in the longitudinal direction, it will tighten or break, reducing the production efficiency of the battery in any case. Let me do it. If the dimensional stability cannot be maintained during the battery runaway as in the latter case, even the shutdown function becomes meaningless, which is a very serious problem. As described above, no technique has been established for producing a separator that does not impair the shutdown function and that can withstand use at high temperatures.

[0008] In the microporous membrane production technology, a microporous membrane precursor is formed by a phase separation process from a composition comprising a polymer and a plasticizer, and a plasticizer extraction / removal process or a stretched thinning process is applied thereto. A technique for forming a microporous film by using the method is known. In such known techniques, depending on whether the stretching step is performed before or after the extraction process, it will be referred to as pre-extraction stretching or post-extraction stretching, respectively.

JP-B-6-21177 and JP-A-6-240036 disclose stretching before extraction and stretching after extraction for the purpose of improving the permeation performance and controlling the pore size of a microporous polyolefin membrane containing an ultrahigh molecular weight component. Are disclosed, but a microporous film having excellent dimensional stability at high temperatures has not yet been obtained. JP-A-6-33653
No. 5 proposes a method for improving the permeation performance and improving the battery safety by using both pre-extraction stretching and post-extraction stretching on a microporous membrane composed of a mixed composition of polyethylene and polypropylene. No attention has been paid to heat shrinkage of the microporous film at high temperatures, and a microporous film having excellent dimensional stability at high temperatures has not been obtained.

[0010]

In the conventional technique for producing a microporous membrane, it has been difficult to freely adjust the permeation performance without impairing the strength of the microporous membrane. That is, in the case of the process using only the stretching before extraction, it is possible to efficiently impart the orientation and increase the strength, but there is a disadvantage that there is no flexibility in terms of the transmission performance. On the other hand, in the case of the process using only the stretching after extraction, the interface breakdown predominantly proceeds by stretching, so that it is possible to obtain a microporous membrane having high permeability, but it is difficult to adjust the permeability.
There is also a disadvantage that the strength is low.

Further, Japanese Patent Publication No. 6-21177, Japanese Patent Application Laid-Open No. 6-240036, and Japanese Patent Application Laid-Open No. 6-3365.
As disclosed in Japanese Patent Publication No. 35, it is possible to improve the permeation performance by using the pre-extraction stretching and the post-extraction stretching together, but it improves the dimensional stability at high temperatures and makes the microporous hard to heat shrink. It is difficult to get a film. Thus, in the art, the establishment of a technique for obtaining a microporous membrane having a permeation performance that can be adjusted over a wide range from a relatively low region to a high region and that has excellent dimensional stability at high temperatures has been left as an issue. .

[0012]

Means for Solving the Problems The present inventor has conducted intensive studies in order to solve the above-mentioned problems, and as a result, by using both pre-extraction stretching and post-extraction stretching and further performing a heat treatment, without impairing the strength, The present inventors have found a method for producing a microporous polyolefin membrane that can freely adjust the permeability and at the same time have excellent dimensional stability at high temperatures, and have accomplished the present invention.

That is, the present invention provides (a) a step of melt-kneading a composition comprising a polyolefin resin and a plasticizer, extruding, cooling and solidifying to form a sheet;
After the step of stretching at least one time in at least one axial direction, (c) a step of extracting the plasticizer after the step b, (d) at least a uniaxial direction after the step c, The present invention relates to a method for producing a microporous polyolefin membrane, comprising a step of performing stretching at least once, and (e) a step of performing a heat treatment following or after the step d.

Further, in the heat treatment step of the above-mentioned production method, the temperature is set to a temperature not lower than 50 ° C. lower than the melting point T _m ° C. of the polyolefin microporous membrane and lower than T _m ° C., and the relaxation rate is set to 1 to 50%. Thermal relaxation is a preferred embodiment of the present invention. In the step (a) of the production method of the present invention, the first method of melt-kneading the polyolefin resin and the plasticizer is as follows. The polyolefin resin is charged into a resin kneading device such as an extruder, and the resin is heated and melted. In this method, a uniform solution is obtained by introducing a plasticizer in a ratio and kneading a composition comprising a resin and a plasticizer. The form of the polyolefin resin to be charged may be any of powder, granule, and pellet. When kneading by such a method, the form of the plasticizer is preferably a liquid at room temperature. As the extruder, a single screw type extruder, a twin screw different direction screw type extruder, a twin screw same direction screw type extruder, or the like can be used.

A second method of melt-kneading a polyolefin resin and a plasticizer is to mix and disperse the resin and the plasticizer in advance at ordinary temperature, and to put the obtained mixed composition into a resin kneading apparatus such as an extruder. This is a method of obtaining a uniform solution by kneading. The form of the mixed composition to be charged may be a slurry when the plasticizer is a liquid at room temperature, and may be a powder when the plasticizer is a solid at room temperature. In the first and second methods, it is important to knead the polyolefin and the plasticizer in a kneading apparatus such as an extruder to obtain a uniform solution, and thereby to improve productivity. it can.

In the step (a) of the production method of the present invention, the first method for producing a sheet-like microporous membrane precursor by extruding and cooling and solidifying is to prepare a homogeneous solution of a resin and a plasticizer with T
This is performed by extruding a sheet through a die or the like, contacting with a heat conductor, and cooling the resin to a temperature sufficiently lower than the crystallization temperature of the resin. The heat conductor used is metal,
Water, air, or the plasticizer itself can be used, but a method of cooling by contacting with a metal roll is particularly preferred because it has the highest heat conduction efficiency. Further, when the sheet is brought into contact with a metal roll, it is preferable to carry out calendering or hot rolling by sandwiching between the rolls, for example, because the efficiency of heat conduction is further increased and the surface smoothness of the sheet is also improved.

A second method for producing a sheet-like microporous membrane precursor is a method in which a homogeneous solution of a resin and a plasticizer is extruded into a cylindrical shape through a circular die or the like, and then processed into a sheet-like shape. In the step (c) of the production method of the present invention, the first method of extracting a plasticizer is to immerse a microporous membrane cut into a predetermined size in a container containing an extraction solvent and sufficiently wash the membrane. The drying is performed by air drying of the attached solvent or drying by hot air. At this time, it is preferable to repeat the immersion operation and the washing operation many times, since the plasticizer remaining in the microporous membrane is reduced. In order to suppress shrinkage of the microporous membrane during a series of operations such as immersion, washing, and drying, it is preferable to restrict the end of the microporous membrane.

A second method for extracting the plasticizer is to continuously feed the microporous membrane into a tank filled with an extraction solvent and immerse the tank in the tank for a sufficient time to remove the plasticizer. And
The drying is then performed by drying the solvent that has adhered.
At this time, a multistage method in which the microporous membrane is sequentially fed to each tank having a concentration difference by dividing the inside of the tank into multiple stages, or a concentration gradient is provided by supplying an extraction solvent from a direction opposite to the running direction of the microporous membrane. It is preferable to apply a known means such as a countercurrent method for increasing the extraction efficiency. In both the first and second methods, it is important to substantially remove the plasticizer from the microporous membrane. Heating the temperature of the extraction solvent within a range lower than the boiling point of the solvent is preferable because the diffusion of the plasticizer and the solvent can be promoted, thereby increasing the extraction efficiency.

In the production method of the present invention, the stretching performed before the extraction step is referred to as “pre-extraction stretching (step (b)”), and at least one stretching operation in at least one axial direction is essential. At least the uniaxial direction refers to uniaxial stretching in the machine direction, uniaxial stretching in the width direction, simultaneous biaxial stretching, and sequential biaxial stretching. The term “at least once” refers to one-stage stretching, multi-stage stretching, and multiple-time stretching.

In the stretching before extraction in the present invention, since the plasticizer is stretched in a state of being dispersed in a high order inside the micropores, crystal gaps, and amorphous portions of the microporous membrane, the stretchability is improved due to the plasticizing effect. At the same time, it has the effect of suppressing an increase in the porosity of the microporous film, and high-stretching can be realized, so that high strength can be achieved. Biaxial stretching is preferable in order to further achieve high strength, and simultaneous biaxial stretching is most preferable because the process can be simplified.

The stretching temperature depends on the melting temperature of the microporous polyolefin membrane.
Point T_mT above 50 ℃ lower than 50 ℃ _mLess than ° C is preferred
And more preferably the melting point T of the microporous polyolefin membrane._m
Over 40 ° C lower than T_mLess than 5 ℃ lower than 5 ℃
Do with. Stretching temperature is T_mLess than 50 ℃ lower than 50 ℃
When stretched, the stretchability deteriorates, and the strain component after stretching remains.
Dimensional stability at high temperatures
No. Stretching temperature is T_mAbove ℃, the microporous membrane melts
It is not preferable because the transmission performance is impaired. Any stretching ratio
Magnification can be set, but preferably 2 to 1 in the axial direction.
20 times, more preferably 4 to 10 times, and biaxial direction
The area magnification is preferably 2 to 400 times, more preferably
Is 4 to 100 times.

In the production method of the present invention, the stretching performed after the extraction step is referred to as post-extraction stretching (step (d)), and at least one stretching operation in at least one axial direction is essential. Since the stretching after the extraction is performed in a state where the plasticizer is substantially removed from the microporous membrane, destruction of the polymer interface predominantly occurs with the stretching, and has an effect of increasing the porosity of the microporous membrane. Therefore, if only stretching after extraction is performed without performing stretching before extraction, which is essential in the present invention, the porosity unnecessarily increases, and stretching orientation cannot be imparted to the microporous membrane, resulting in low strength. Will be.

On the other hand, the production method of the present invention in which stretching before extraction and stretching after extraction are used together is useful because the porosity can be increased without impairing the strength of the microporous membrane. The stretching temperature is preferably T _m lower than ° C. 50 ° C. temperature lower or higher than the melting point T _m ° C. microporous polyolefin membrane, more preferably a melting point T _m of a polyolefin microporous membrane
The process is performed at a temperature lower than 40 ° C. and lower than T _m 5 ° C. by 5 ° C. or lower. If the stretching temperature is lower than the temperature lower by 50 ° C. than T _m ° C., the stretchability deteriorates, and a strain component after stretching remains,
It is not preferable because dimensional stability at high temperature is reduced.
If the stretching temperature is higher than T _m ° C, the microporous membrane is melted and the permeability is impaired, which is not preferred. The stretching ratio can be set to any ratio, but is preferably within 5 times in uniaxial direction and within 20 times in biaxial direction.

In the production method of the present invention, the heat treatment (step (e)) is performed subsequent to or subsequent to the stretching after extraction, and refers to either heat setting or thermal relaxation. . The heat setting means a step of maintaining a set stretching ratio at the time of stretching after extraction or performing a heat treatment in a tensioned state while being restrained. (A) On the other hand, the thermal relaxation refers to a state of relaxation. Means a step of performing a heat treatment. Both heat fixation and thermal relaxation remove residual stress and strain components that are considered to occur during stretching, increase dimensional stability at high temperatures, and adjust permeability performance represented by porosity and air permeability. It has. The first embodiment of the heat treatment is performed continuously after the extraction and subsequent to the stretching. For example, after performing stretching with a uniaxial or biaxial stretching machine such as a tenter, the maximum set stretching ratio at the time of stretching is set. This is a method in which a heat treatment is performed for a predetermined time while maintaining the temperature or relaxing the film by setting a magnification smaller than the maximum set stretching magnification. The second embodiment of the heat treatment is performed intermittently after stretching after extraction, for example, after stretching with a test biaxial stretching machine such as a stretcher, the microporous membrane is restrained again. In this method, the heat treatment is performed for a predetermined period of time, or the heat treatment is performed while relaxing the temperature by setting the magnification to a value smaller than the set magnification at the time of restraint.

The relaxation rate in the production method of the present invention means a rate of thermal relaxation set in the heat treatment step, preferably 1 to 50%, more preferably 10 to 50%.
40%. If the relaxation rate is less than 1%, especially 0%,
In the present invention, referred to as heat fixing, in this case, the dimensional stability at high temperatures of the microporous membrane tends to be relatively poor,
There is a fear that a long heat treatment is required and production efficiency is reduced. On the other hand, if the relaxation rate exceeds 50%, there is a fear that wrinkles and film thickness distribution may be caused. Therefore, in the present invention, it is preferable to set the relaxation rate within the range of 1 to 50% and to relax the heat.

In the production method of the present invention, as long as the advantages of the present invention are not impaired, that is, in order to improve the dimensional stability at a high temperature and to adjust the permeation performance, the degree is not impaired. Post-processing may be performed. Examples of the post-treatment include a hydrophilic treatment with a surfactant or the like and a cross-linking treatment with ionizing radiation or the like. Polyolefin resin used in the present invention, the usual extrusion,
Refers to resin used for injection, inflation, and blow molding, and uses homopolymers and copolymers of ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Can be. Further, a polyolefin selected from the group of these homopolymers and copolymers can be used as a mixture. Representative examples of the polymer include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultra high molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, polybutene, ethylene propylene rubber, and the like. . When the microporous membrane obtained by the production method of the present invention is used as a battery separator, it is a low-melting resin, and from the performance required for high strength, it is particularly preferable to use a resin mainly composed of high-density polyethylene. .

The average molecular weight of the polyolefin resin used in the present invention is preferably from 50,000 to less than 1,000,000,
More preferably, it is 100,000 or more and less than 700,000, and most preferably 200,000 or more and less than 500,000. The average molecular weight refers to a weight average molecular weight obtained by GPC (gel permeation chromatography) measurement or the like, but it is generally difficult to accurately measure a resin having an average molecular weight exceeding 1,000,000. Therefore, a viscosity average molecular weight by a viscosity method can be used as a substitute. If the average molecular weight is smaller than 50,000, the melt tension during melt molding is lost and moldability is deteriorated, and stretchability is deteriorated and strength is unfavorably lowered. If the average molecular weight exceeds 1,000,000, it tends to be difficult to obtain a uniform resin composition.

The molecular weight distribution of the polyolefin resin used in the present invention is preferably 1 or more and less than 10, more preferably 2 or more and less than 9, and most preferably 3 or more and less than 8. The molecular weight distribution is represented by the ratio ( _Mw / _Mn ) between the weight average molecular weight ( _Mw ) and the number average molecular weight ( _Mn ) obtained by GPC measurement. If the molecular weight distribution exceeds 10, the stretchability tends to be poor, and there is a possibility that the local distribution of the film thickness or the strength may be reduced.

As the plasticizer used in the present invention,
Any non-volatile solvent that can form a uniform solution at a temperature equal to or higher than the melting point of the polyolefin resin when mixed with the polyolefin resin may be used. Examples include hydrocarbons such as liquid paraffin and paraffin wax, esters such as dioctyl phthalate and dibutyl phthalate, and higher alcohols such as oleyl alcohol and stearyl alcohol.

The ratio of the polyolefin resin to the plasticizer used in the present invention is a ratio sufficient to cause microphase separation and to form a sheet-like microporous membrane precursor, and impair productivity. If it is not enough. Specifically, the weight fraction of the polyolefin resin in the composition comprising the polyolefin resin and the plasticizer is preferably 20 to 70%, and more preferably 30 to 60%. If the weight fraction of the polyolefin resin is less than 20%, the melt tension at the time of melt molding becomes insufficient, resulting in poor moldability. Weight fraction of polyolefin resin is 2
It is also possible to carry out at a ratio of less than 0%, but in this case, in order to increase the melt tension, it becomes necessary to mix a large amount of ultrahigh molecular weight polyolefin,
It is not preferable because the uniform dispersibility decreases.

The extraction solvent used in the present invention is preferably a poor solvent for the polyolefin and a good solvent for the plasticizer, and has a boiling point lower than the melting point of the microporous polyolefin membrane. As such an extraction solvent,
For example, hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride and 1,1,1-trichloroethane, alcohols such as ethanol and isopropanol, ethers such as diethyl ether and tetrahydrofuran, acetone, Ketones such as 2-butanone are exemplified. Further, in consideration of environmental adaptability, safety and hygiene, alcohols and ketones are preferable among the solvents.

The composition used in the present invention may further contain an antioxidant, a nucleating agent, an antistatic agent,
Additives such as a flame retardant, a lubricant, and an ultraviolet absorber may be mixed. The microporous membrane of the present invention refers to a porous sheet or film substantially composed of polyolefin, and is used, for example, as a battery material for a separator or the like. The form of the battery is not particularly limited, and is suitable for use in, for example, a cylindrical battery, a square battery, a thin battery, a button battery, an electrolytic capacitor, and the like.

When a microporous membrane is produced by using the production method of the present invention, the thickness of the microporous membrane is preferably from 1 to 500 μm, more preferably from 10 to 100 μm. If the film thickness is smaller than 1 μm, the mechanical strength becomes insufficient, and if it is larger than 500 μm, the volume occupied by the separator increases, which is disadvantageous in increasing the capacity of the battery, which is not preferable.

When a microporous membrane is manufactured using the manufacturing method of the present invention, the air permeability of the microporous membrane is 3000 seconds / 100 c
c / 25 μm or less, 1000 sec / 1
More preferably, it is not more than 00 cc / 25 μm. The air permeability is defined by the ratio between the air transmission time and the film thickness.
If the air permeability is more than 3000 seconds / 100 cc / 25 μm, the ion permeability deteriorates or the pore size becomes extremely small, which is not preferable from the viewpoint of permeability.

When a microporous membrane is produced by using the production method of the present invention, the porosity of the microporous membrane is preferably from 20 to 80%, more preferably from 30 to 60%.
If the porosity is less than 20%, ion permeability such as air permeability and electric resistance becomes insufficient, and if it is more than 80%, strength such as piercing strength and tensile strength becomes insufficient.

When a microporous membrane is produced by the production method of the present invention, the piercing strength of the microporous membrane is 300 g / 25.
μm or more, more preferably 400 g / 25 μm or more. The piercing strength is defined by the ratio between the maximum load and the film thickness in the piercing test. If the piercing strength is less than 300 g / 25 μm,
When winding the battery, defects such as short-circuit failure increase, which is not preferable.

When a microporous film is manufactured by using the manufacturing method of the present invention, the heat shrinkage of the microporous film is an index for evaluating the dimensional stability of the microporous film at a high temperature, and is measured in the machine direction of the microporous film. Or, in the width direction, preferably 20% or less, more preferably 10% or less, and most preferably 5% or less.
It is preferable to set the following. If the heat shrinkage exceeds 20%, it may cause a safety trouble such as a short circuit inside the battery, which is not preferable.

[0038]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to examples. The test method shown in the examples is as follows. (1) Film thickness dial gauge (PEACOCK NO.
25). (2) Porosity A 20 cm square sample was cut from the microporous membrane, and the volume (cm ³ ) and weight (g) of the sample were measured. From the obtained results, the porosity (%) was calculated using the following equation. Calculated.

Porosity = 100 × (1−weight ÷ (density of resin × volume)) (3) Air permeability Air permeability measured by a Gurley air permeability meter according to JIS P-8117. From the time (seconds / 100 cc) and the film thickness (μm), the film thickness was converted according to the following equation to obtain the air permeability (seconds / 100 cc / 25 μm).

Air permeability = air permeability time × 25 ÷ film thickness (4) Piercing strength Using a compression tester (KES-G5 manufactured by Kato Tech),
A piercing test is performed under the conditions of a radius of curvature of the needle tip of 0.5 mm and a piercing speed of 2 mm / sec, and a maximum piercing load (g).
The film thickness was converted from the film thickness and the film thickness (μm) according to the following formula to obtain the piercing strength (g / 25 μm).

Puncture strength = maximum puncture load × 25 ÷ film thickness (5) Thermal shrinkage A sample of 20 cm square is cut from the microporous membrane, and the hot air circulation oven heated to 100 ° C. without restraining all sides of the sample. And heat-treated for 2 hours. The dimensions in the machine direction and the width direction of the microporous membrane were measured before and after heating, and the heat shrinkage (%) was calculated according to the following equation.

Heat shrinkage = 100 × (1- (dimension after heating / dimension before heating)) (6) Average molecular weight and molecular weight distribution GPC (gel permeation chromatography) was measured under the following conditions, and the weight was measured. The average molecular weight (M _w ) and number average molecular weight (M _n ) were determined, and the average molecular weight was M _w
And _Mw / _Mn for molecular weight distribution. Equipment: WATERS 150-GPC Temperature: 140 ° C Solvent: 1,2,4-trichlorobenzene Concentration: 0.05% (injection volume: 500 μl) Column: Shodex GPC AT-807 / S 1
This, Tosoh TSK-GELGMH ₆ -HT 2 pieces Dissolution condition: 160 ° C., 2.5 hours Calibration curve: Calculated in third order using a polyethylene conversion constant of 0.48 for a polystyrene standard sample (7) Melting point Seiko Denshi Kogyo, Differential Scanning Calorimeter DSC-2
Using the sample No. 10, about 7 mg of the sample was placed under a nitrogen stream and evaluated from the endothermic peak temperature when the temperature was raised from room temperature at a rate of 10 ° C./min. (8) Relaxation rate The relaxation rate (%) is defined as the following equation based on the difference between the set magnification at the time of stretching after extraction and the set magnification at the time of heat treatment with respect to the dimensions of the microporous membrane before extraction and stretching. did.

Relaxation rate = 100 × (stretching setting ratio after extraction−
Heat treatment setting magnification)

[0044]

Example 1 High-density polyethylene (weight average molecular weight 25
, Molecular weight distribution 7, density 0.956) and 0.3 parts by weight of 2,6-di-t-butyl-
p-Cresol was dry-blended using a Henschel mixer and charged into a 35 mm twin screw extruder. Further, liquid paraffin (a kinematic viscosity at 37.78 ° C. of 7
5.9 cSt), melt-kneaded at 200 ° C., and extruded through a coat hanger die onto a cooling roll controlled at a surface temperature of 40 ° C. to obtain a thickness of 1.8 m.
m of sheets were obtained. Here, the composition ratio was adjusted such that liquid paraffin was 55 parts by weight with respect to 45 parts by weight of polyethylene. The obtained sheet was stretched before extraction using a simultaneous biaxial tenter stretching machine, then immersed in methylene chloride to extract and remove the liquid paraffin, and then the attached methylene chloride was dried and removed. Further, the film was extracted and stretched in the width direction using a tenter stretching machine, and then heat-treated while relaxing in the width direction. The molding conditions are shown in Table 1, and the physical properties of the obtained microporous membrane are shown in Table 2. The melting point of the obtained microporous membrane was 133.9 ° C.

[0045]

Example 2 A microporous film was obtained in the same manner as in Example 1 except that the conditions of the heat treatment were changed to the conditions shown in Table 1. Table 2 shows the physical properties of the obtained microporous membrane.

[0046]

Example 3 A microporous membrane was obtained in the same manner as in Example 1 except that the conditions of stretching after extraction and heat treatment were changed to those shown in Table 1. Table 2 shows the physical properties of the obtained microporous membrane.

[0047]

Comparative Example 1 A microporous film was obtained in the same manner as in Example 1 except that no heat treatment was performed. Table 2 shows the physical properties of the obtained microporous membrane.

[0048]

Example 4 In Example 1, the microporous membrane after stretching after extraction was cut into a square, fixed to a frame again, and heat-fixed for 1 minute under the conditions shown in Table 3 to obtain a microporous membrane. Was. Table 4 shows the physical properties of the obtained microporous membrane.

[0049]

Example 5 A sheet obtained in the same manner as in Example 1 was stretched at a stretching temperature of 120 ° C. using a test biaxial stretching machine.
, A sequential biaxial stretching of 7 times in the machine direction and 7 times in the width direction was performed. Subsequently, it was immersed in methylene chloride to extract and remove the liquid paraffin, and then the attached methylene chloride was dried and removed. Further, stretching was performed after extraction 1.5 times in the width direction at a stretching temperature of 115 ° C., and heat treatment was performed while relaxing in the width direction at a heat treatment temperature of 120 ° C. and a relaxation rate of 10%. The heat shrinkage in the width direction of the obtained microporous film was 8%.

[0050]

Example 6 34 parts by weight of high-density polyethylene used in Example 1, 6 parts by weight of linear copolymerized polyethylene (melt index 0.017, density 0.929, propylene content 1.6 mol%), liquid paraffin 60 parts by weight, and 0.3 parts by weight of 2,
6-Di-t-butyl-p-cresol was mixed,
Injected into Toyo Seiki Labo Plast Mill, 200
The mixture was melt-kneaded at ℃. Subsequently, after forming into a sheet using a compression molding machine heated to 200 ° C., the mixture was cooled and solidified using a water-cooled compression molding machine to obtain a sheet having a thickness of 1.3 mm. The obtained sheet was simultaneously biaxially stretched 4 × 4 times at a stretching temperature of 120 ° C. using a test biaxial stretching machine. Subsequently, it was immersed in 2-butanone to extract and remove liquid paraffin, and then attached 2-butanone was dried and removed. Further, 2 × 2 simultaneous biaxial stretching was performed at a stretching temperature of 115 ° C., and a heat treatment was performed while relaxing in the width direction at a heat treatment temperature of 120 ° C. and a relaxation rate of 10%. The melting point of the obtained microporous membrane was 130.7 ° C. The physical properties of the microporous film were 40 μm in film thickness, porosity was 64%, and air permeability was 100 seconds.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

[Table 3]

[0054]

[Table 4]

[0055]

According to the method for producing a microporous polyolefin membrane of the present invention, dimensional stability at a high temperature can be improved, and at the same time, there is a flexible aspect in which permeability can be freely adjusted. In addition, wrinkles of the microporous membrane,
It is possible to prevent defects such as looseness and film thickness distribution from deteriorating the quality. Thus, the microporous membrane produced by the present invention has high battery safety, especially when used as a battery separator, and also has various permeability properties to meet diversifying battery needs. A membrane can be manufactured.

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Claims (2)

[Claims] (A) a step of melt-kneading a composition comprising a polyolefin resin and a plasticizer, extruding, cooling and solidifying to form a sheet, and (b) at least one axial direction after the step (a). A step of performing one stretching,
(C) a step of extracting the plasticizer after the step b;
(D) A method for producing a microporous polyolefin membrane, comprising a step of performing at least one stretching in at least one axial direction after the step c, and a step of performing a heat treatment following or after the step d. 2. In the heat treatment step, heat is relaxed by setting the temperature to a temperature not lower than T _m ° C. which is 50 ° C. lower than the melting point T _m ° C. of the polyolefin microporous membrane and a relaxation rate of 1 to 50%. The method for producing a microporous polyolefin membrane according to claim 1, wherein: